In many optical applications, electromagnetic radiation is transmitted along optical fibres and must therefore be coupled into the fibre at one end (for example from a laser source) and coupled out of the fibre at the other end into, for example, another optical fibre or some other optical component. In order to do this the radiation is typically focused on to the fibre end by one or more lenses, and lenses are similarly provided at the other end of the fibre to focus or collimate the exiting radiation.
One problem with such an arrangement is that both lateral misalignment between the optical axis of the fibre and the optical axis of the lens (or other optical component) and relative tilt between the central axes of the lens and fibre can lead to energy losses as the radiation is coupled into or out of the optical fibre.
In order to help reduce losses due to poor alignment between the fibre and lens (or other optical element), the end of the optical fibre and the lens are typically both mounted in a cylindrical tube in a predetermined fixed alignment with each other. This assembly is usually known as a lens barrel or a lens tube. The lens barrel holds the lens and fibre in a fixed, aligned relationship, thereby helping to avoid losses due to misalignment between the lens and fibre in use.
However, even with such an arrangement, losses can still occur due to misalignment between the fixed fibre and lens in the lens barrel and the laser source or other optical component to which the optical fibre is being optically coupled.
For example, any lateral displacement of the fibre and lens with respect to the incident radiation will cause the radiation impinging on the fibre end to be incident at an angle to the optical axis of the fibre. This could mean that some of the incident radiation falls outside the acceptance angle of the fibre, thereby leading to losses.
Furthermore, any tilt of the central axis of the radiation source with respect to the central axis of the fibre and lens arrangement will cause the focused radiation to be displaced across the end face of the fibre. This may result in some radiation missing the fibre end face.
These situations can be common when coupling radiation from a laser or other sources into an optical fibre, and in both cases, the coupling efficiency will be degraded.
A similar situation can arise with the radiation emerging from an optical fibre where alignment with other optical components is required, or radiation needs to be coupled from one optical fibre to another (either with or without additional components in between).
In order to reduce these alignment losses, the relative positions of the lens barrel and the laser source or other optical component to which the optical fibre is being coupled are often adjusted prior to use to try to optimise their alignment. In order to achieve this, the lens barrel is typically coupled to the laser source or optical component by means of a connector which allows the position of the lens barrel (and thus the fibre and lens) with respect to the laser source or optical component to be adjusted. Such a connector would typically provide adjustment of both tilt and lateral displacement, although the adjustable parameters may vary depending upon the application concerned.
A known prior art optical fibre connector is shown in FIG. 1. It comprises a cylindrical tube 1 which can receive a lens barrel 2 in use. The tube has two pairs of adjustment screws 3a, 3b and 4a, 4b, which extend through the tube and can engage the outer surface of the lens barrel once it has been inserted for adjusting the position of the lens barrel within the tube. An adjustable spring 5 is provided in an opposed relationship to the screws 3a, 3b, 4a, 4b to resiliently bias the lens barrel into engagement with the tips of the adjustment screws. The connector can be fixed to the optical component, laser source, etc., to which the optical fibre 6 is to be coupled by means of a flange 7. In use, the lens barrel is inserted into the tube 1, and the spring 5 adjusted to resiliently bias it against the adjustment screws 3a, 3b, 4a, 4b. The screws 3a and 4a, or 3b and 4b can then be moved together to move the lens barrel laterally (i.e. in a plane perpendicular to the axis of the lens barrel), or the screws 3a and 3b, or 4a and 4b can be moved together to tilt the lens barrel, in order to align the lens and optical fibre with the laser source, optical component, etc.
The Applicants have already proposed an improved version of this type of prior art connector in their UK Patent No. 2325058 which, inter alia, helps to avoid crosstalk between adjustment movements of the adjustment screws of the connector and facilitates more consistent rotational orientation of the lens barrel in the connector (with respect to rotation about its longitudinal axis).
However, the Applicants have now recognised that another important factor affecting the coupling efficiency and operation, etc., when coupling electromagnetic radiation into and out of optical fibres is the polarisation state of the radiation. This is because many optical systems are polarisation sensitive and may in particular require highly plane polarised radiation to operate properly or efficiently. For example, an optical system may include polarisation sensitive materials, such as coatings on components such as lenses. In such a system, if the incident radiation is not highly plane polarised and appropriately aligned with, e.g., the polarisation axis of the polarisation sensitive component, then that could lead to transmission losses and possibly localised heating.
It can also be the case that materials being tested in an optical system can be polarisation sensitive (such that insufficiently plane polarised radiation and incorrect polarisation alignment can lead to inaccurate measurements) or indeed that the type of measurement itself is polarisation sensitive (e.g. where a scattering detection is being made at 90E to the incident radiation's direction of travel).
In such polarisation sensitive systems it is desirable therefore to try to ensure that the electromagnetic radiation exiting an optical fibre (and then, e.g., entering a polarisation sensitive component of the system) exhibits a high degree of (plane) polarisation (i.e. has a high extinction ratio, typically better than 100:1). One way to achieve this would be to use so-called polarising optical fibres which only emit plane polarised light (regardless of the incident radiation entering them). However, such fibres are difficult to manufacture and hence can be expensive and difficult to obtain.
It is also known to use polarisation maintaining optical fibres. These fibres are birefringent, and have two (orthogonal) polarisation axes, a so-called “fast” axis and a “slow” axis. If they receive incident plane polarised electromagnetic radiation that is accurately aligned with one of their polarisation axes (in practice usually the slow axis, although the fast axis can be used), polarisation maintaining optical fibres will maintain the plane polarisation of the incident radiation and emit plane polarised electromagnetic radiation. Thus, by using plane polarised incident radiation and a polarisation maintaining optical fibre, plane polarised output radiation can be delivered to optical components of an optical system. As lasers typically emit plane polarised electromagnetic radiation light, the use of polarisation maintaining fibres in combination with lasers is in fact a convenient way of delivering plane polarised radiation in an optical system.
However, the Applicants have recognised that a problem with this type of arrangement is that the polarisation axis of the incident plane polarised radiation (e.g. from the laser) must be accurately aligned with the polarisation axis of the polarisation maintaining optical fibre, or otherwise the polarisation of the exiting radiation can become unstable and can fluctuate in use depending on, e.g., environmental conditions. This can have the effect, e.g., that the exiting radiation is no longer highly plane polarised.
The reason for this is that the birefringent polarisation maintaining optical fibre can accept orthogonal polarisation planes and so if the incident radiation is not accurately aligned with one or other orthogonal axis of the optical fibre, the radiation will in fact travel down the optical fibre as two orthogonal components. This may not be a problem where the orthogonal components are in phase when they exit the fibre (and so interfere constructively on exiting the fibre), but if a phase difference between the orthogonal components is present as the components exit the fibre, then the polarisation state of the exiting radiation will no longer be highly plane polarised, but will only be partially polarised (and exhibit a lower extinction ratio).
The phase difference, if any, as the orthogonal components exit the fibre depends, inter alia, on the length of the fibre (because each component travels a different refractive index path (and hence effective path length) through the fibre). Thus only certain fibre lengths will ensure that the two components are brought to the same phase as they exit the fibre. Furthermore, the effective refractive index and path length encountered by each orthogonal component can also be affected by e.g., environmental factors such as stresses or temperature changes in the fibre, which factors may furthermore affect the effective refractive index and path length encountered by each orthogonal component differently. Thus even if the fibre length is selected such that the orthogonal components should exit fibre in phase, environmental factors can introduce (effectively random) phase differences that will alter the phase relationship.
The effect of this overall is that when using polarisation maintaining optical fibres, unless the plane of the incident polarised electromagnetic radiation (light) is accurately aligned with a polarisation axis of the polarisation maintaining optical fibre, then the polarisation of the radiation exiting the fibre can be unstable (such that, e.g., the extinction ratio can vary) and can change with, e.g., environmental factors.
It can be important therefore in polarisation sensitive optical systems that use polarisation maintaining optical fibres to ensure accurate alignment between incident plane polarised radiation and the polarisation axis of the polarisation maintaining optical fibre. This essentially means that the rotational alignment of the optical fibre (with respect to rotation about its optical axis) must be set accurately in relation to the optical axis of the incident radiation.
As discussed above, existing optical fibre connectors and in particular the Applicant's connector described in their UK Patent No. 2325058 can help to achieve more accurate rotational alignment (and hence polarisation alignment) between an optical fibre and, e.g., a laser source. However, the Applicants have found that even with such connectors sufficiently accurate polarisation alignment can still be difficult to achieve.
For example, when fixing the connector to the laser source it can be difficult to ensure reliable alignment between, e.g., a reference axis of the connector and the polarisation axis of the laser radiation. This may be due to, e.g., tolerances in the fixings (e.g., flange and screws) used to fix the connector to the laser source and/or misalignment between the fixing points for the connector provided on the laser itself and the polarisation axis of the laser radiation. While it may then be possible in use to adjust the rotational orientation of an optical fibre in the connector to achieve correct alignment of the polarisation axes of the laser beam and optical fibre, such adjustment can be time consuming and difficult to do and may have to be repeated every time a fibre is placed in the connector (even if the alignment of the connector to the laser remains unchanged). It may also be necessary to carry out such adjustments every time the connector is connected to a different laser.